Known in the art is a method for anti-corrosion cathodic protection of an elongated metal object, in which a long-line anode in the form of a continuous flexible steel core in an electrically conductive polymer envelope is installed in an electrolytic medium near the surface to be protected. In this case, the anode is disposed along the object at a pre-set distance therefrom determined by the thickness of the electric insulation plate between the anode and the surface to be protected, then the object and anode are connected to a polarizing current source (U.S. Pat. No. 4,487,676).
This known method however has a number of significant drawbacks. Thus, the anode is disposed in the immediate vicinity of the surface to be protected, the distance between them is hot optimized with respect to the electrical characteristics of the whole system. This fact, even in the case of a plane-parallel electric field, reduces the protection and results in nonuniform distribution of potential, especially with aged insulation.
Furthermore, the prior art method of disposition of the protective grounding (anode) is associated with a danger of over-protection at the drain point, i.e. there is a danger that the whole protection system will more rapidly fail.
Attempts to avoid over-protection by reducing the potential have resulted in reduction of the protection zone, i.e. impairment of the protection efficiency as a whole.
Known in the art is a method of cathodic protection of extended objects by means of a flexible long-line anode, which provides an optimum distance between the anode and the surface to be protected. The known method includes installation of a long-line anode in the form of a continuous flexible metal core encased by an electrically conductive flexible polymer envelope in contact therewith and installed in an electrolytic medium at a preset distance from the object, connection of this object and anode to current sources and polarization of the object from the anode. According to this method, the anode material resistance must be within 0.1 to 1000 ohm cm, while its longitudinal resistance must not exceed 0.03 to 0.003 ohm m. In so doing, the anode must be arranged relative to the object to be protected so as to keep a ratio (b+D)/(a+D)&lt;2, where a is the minimum distance between the anode and the object to be protected, b is the maximum distance between the anode and the object to be protected and D is the maximum linear size of the object to be protected in the direction normal to the anode axis (U.S. Pat. No. 4,502,929).
This method is still characterized by some drawbacks hindering its application. For example, the known method does not provide needed uniformity of distribution of the protective difference of potentials along the circumference of the insulated pipe in the process of long-term operation. A similar negative result occurs when the pipe surface has no installation. This is due to the fact that the protective difference of potentials includes both the pipe potential proper determined by the integral value of the linear density of the polarizing current and the potential of the surrounding medium depending on the differential densities of the current flowing at each point of the volume of the current-conductive space. Under otherwise equal conditions, the latter is substantially determined from not only the ratio of the distances between the anode and the object to be protected and the linear size of the latter but also depends on the disposition of damage and discontinuities in the insulation along the pipe circumference and the electrochemical properties of the surrounding ground.
In many cases, with the ratio (b+D)/(a+D)&lt;2, it is not possible to ensure the required level of protection over the whole surface, e.g. a single cross-section of a pipeline. Indeed, in the case of cathodic protection of adjacent sections of a pipeline 1400 mm in diameter with an insulation resistance of 300 ohm m and 1000 ohm m the ratio of the densities of the cathodic polarizable current must meet the ratio of 3:1 in order to provide a uniform protective potential. In this case the potentials of the nearest point of the ground near the pipeline with the same departure of the anode will also meet this ratio. Assuming that b&lt;&lt;D and a&lt;&lt;D under condition that b/a&lt;2, it is impossible to compensate the nonuniformity of the potentials of the ground points and, therefore, also the level of protection of adjacent sections characterized by K=3.
A similar situation is valid when a homogeneous section of a pipeline is to be protected. In this version the ground potential at the near and remote generatrix lines of the pipe remains nonuniform and this results in nonuniformity of distribution of the protective potential difference over the circumference and reduces the level of protection. The limited ratio does not allow this nonuniformity to be avoided because for pipelines under the condition that b-a=D it assumes a form of a/D&gt;0, which makes the condition of attaining uniformity of the level of protection indefinite.
The field of application of the method is also limited by the predetermined therein ranges of the resistance of the anode material, as well as that of the structure as a whole. In these ranges the anode cross section (taking no account of the flexible core) must be at least 0.33-333 m.sup.2 (with a diameter of 0.63-18.3 m), and this is completely unreal. If no account is taken of the limiting values of the longitudinal resistance of the core (0.03 to 0.0003 ohm cm) specified in the description, its diameter should be in the range of 0.9 to 8.7 mm which is also unlikely taking into account the technology of manufacture and application of the anode, since this makes it less strong or flexible.
Since the attainment of a required level of protection depends in general on the absolute value of the protection current and the rate of attentuation of the current along the anode, the application of the prior art method can be inefficient in high-resistance grounds due to an increase of the input resistance of the anode or in connection with good condition of the insulation coating of the object to be protected. In these cases, it will be impossible to obtain the required value of the protection current due to the high contact resistance of the anode and distribution of the required density of the protection current due to a high value of the constant of propagation of the current along the anode. Both these factors essentially limit the field of effective application of the known extended anodes in general and of the above method in particular.
Taking into account the peculiarities of the electrochemical processes taking place in ground electrolytes, the basic requirements to the grounding electrodes are their low rate of solubility, particularly of the anode, flow resistance to the current flow and uniform current yield of the working surface of the electrode. The fulfillment of the above requirements provides longevity and operational efficiency of the electrode. At the same time, conditions of cyclic transportation and assembly loads require that the electrodes should have as much flexibility and elasticity as possible to enhance their operational reliability.
With cathodic protection of extended structures the design of cable type electrodes (extended electrodes) are advantageous over pin type electrodes since the current yield of the extended electrodes is effected in a plane-parallel field providing high efficiency of the protection.
Known in the art is a grounding electrode used in cathodic protection systems which is made in the form of a plurality of working elements (iron-silicon anodes) distributed along a current-conducting power cable and electrically connected thereto by contact units of a special design providing continuity of the cable and monolithic structure of the electrode as a whole. Each working element of the electrode comprises a body with a central hole having a conical section, a continuous power cable put through the hole in the electrode body and a means for fixing the electrode body to the cable and simultaneously providing an electric contact therewith. The means for fixing and electric contact is made in the form of two semi-envelopes encompassing the cable and disposed in the hole of the electrode body. The semi-envelopes have a central portion made of an electrically conductive material in direct contact with the bare cable and two end conical sleeves made of an elastic dielectric material. The semi-envelopes of the fixing means are distributed in pairs along the cable axis and form a monolithic connection of the electrode elements using the wedge method (U.S. Pat. No. 3,326,791).
The use of iron-silicon anodes as working elements leads to electrode brittleness and significant losses during transportation and assembly.
The contact units with conical dielectric sleeves do not provide reliable enough contact due to their possible mechanical deformation during transportation and assembly. In addition, such units do not allow protection of the current-conductive cable against direct electric contact with an electromagnetic medium and this results in premature destruction of the electrode and its failure. As a result the life of such electrodes is short.
Known in the art is a flexible extended anode for cathodic protection against corrosion of the internal surface of a tank made of a magnetically perceptive metal with an electrolytic medium. The anode comprises at least one steel mainline conductor, a flexible extended envelope made of an electrically-conductive polymer encompassing the conductor and having an electric contact with it, and a flexible dielectric layer of a magnetic material (permanent magnet) connected along the anode axis with the envelope mechanically or through an adhesive layer.
The magnetic dielectric layer maintains the anode near the surface to be protected but excludes its electric contact with the envelope. A layer of porous material (additional porous envelope) is disposed between the electrically conductive polymer envelope of the anode (U.S. Pat. No. 4,487,676).
The known anode does not allow the current distribution to be controlled when protecting tanks or other objects of a similar shape, i.e. with discretely differential quality of the surface state. The anode is limited along the length of the protection zone due to non-compensated attenuation of the current in the monolithic electrically-conductive envelope and is limited by zone of protective effect (on both sides of the anode) due to the disposition of the anode directly on the surface to be protected as is necessary for the magnetic dielectric layer. In connection with these drawbacks, in order to guarantee a required level of protection over the entire surface to be protected, the anode must operate under high current loads which results in premature wear and consequently in a reduction of service life.
The solution which is closest to the claimed one in its technical essence is an extended flexible electrode of an electrically-conductive polymer composition used in systems of cathodic protection of metal objects, e.g. pipelines. The electrode is made in the form of a band and comprises an extended flexible metal core and an evelope of an electrically-conductive polymer based on thermoelastoplastic materials or plastic materials of the polyvinyl chloride type encompassing the core in electric contact therewith and forming a working, electrochemically active surface of the electrode. The electrode may be disposed in an additional external dielectric electrolytically impermeable envelope preventing direct contact of the electrode working surface with the object surface (GB, A, 2,100,290).
The electrode does not have adequate reliability, especially during assembly due to its low elasticity and frost resistance, since at a temperature of below -10.degree. to -15.degree. C. the envelope material starts cracking. These properties of the electrode also have an adverse effect on its life. In addition, the electrode life is low due to its liability to biological destruction due to a low content of a filler in the envelope material; rapid workout of the filler opens access of the elctrolyte to the core, which results in accelerated work-out, which is also a result of a low content of plasticizer (washing out of the plasticizer and quick cracking of the electrode envelope) caused by low material capacity of the thermoelastoplastic materials and plastics used in the envelope material.
Furthermore, the electrode design permits use of a current-conductive core with a rated resistance of 0.5 ohm mm.sup.2 /m (for comparison, the resistance of a copper core is 0.018 ohm mm.sup.2 /m while that of the steel core is 0.24 ohm mm.sup.2 /m). This requires a minimum diameter of 4.5 mm with the worst permissible resistance of 0.03 ohm/m. At the same time, the realization of the best resistance of 0.0003 ohm/m is practically impossible since it is realizable with a diameter of 45 mm. At the same time, the resistance of the material of the polymer envelope does not exceed 10 ohm m. This does not make it possible to completely utilize the advantages of the extended electrode provided by its constant current attenuation whose minimum value is 5.5 10.sup.-3 l/m. Under such conditions the current load on the electrode increases, especially near the point of its connection and this also reduces the electrode life.
The electrically-conductive polymer compositions and electric devices built around them are well known in the art. The main components of such compositions are carbon-containing fillers (elementary carbon) and a polymer matrix or binder while the properties of each composition are modified by introducing various additives depending on the designation and conditions of application of the composition (U.S. Pat. No. 4,442,139).
The main requirements to the composition for grounding electrodes consist of high electrical conductivity and low rate of solubility in an electrolytic medium. The conditions of transportation and storage as well as the technology of assembly of the grounding electrodes require their high elasticity.
With respect to the elasticity characteristic the electrodes based on electrically-conductive polymers are advantageous over for example electrodes based on metal-oxide or iron-silicon mass used in cathodic protection of metal structures.
However, stable combination of a high elasticity index (minimum 10%) with optimum for the given type of electrolyte (e.g. ground) indexes of electrical conductivity and solubility (in particular, anode) is a complex technical problem.
An electrically-conductive composition is known having high electrical conductivity which comprises an electrically-conductive filler (metal powder plus gas soot) and a dispersing component somewhat compatible with rubber, e.g. polyvinyl chloride, polystyrene, nylon, polyethylene glycol taken in a weight ratio 40-60 and 60-40 respectively to form a mixture with an elastomer binder such as natural rubber, polybutadiene, polyisoprene, ethylene-propylene rubber copolymers. The ratio of the filler with a dispersing agent and a rubber base of the matrix in the composition is from 1.1:1 to 5:1 (U.S. Pat. No. 4,642,202).
The known composition has a specific resistance less than 10.sup.6 ohm cm with low concentrations of the electrically-conductive filler.
However, from the point of view of its possible application for grounding electrodes, in particular, for the anode grounders in the system of cathodic protection, it has a number of significant drawbacks. First, the plastics, like polyvinyl chloride and polystyrene, included in the composition feature reversibility of deformation, which makes the composition inadequately elastic, particularly at low temperatures. Furthermore, the compositions based on plastic materials of the polyvinyl chloride type have low solid matter content, i.e. low filler content. On the other hand, the metal powder-filler causes drastic oxidation of the polymer, particularly under the effect of the applied current, and this leads to cracking of the polymer and to loss of elasticity.
The electrolyte penetrating through the pores and microscopic cracks causes dissolving of the metal and fast washout of the filler, which with a low content of the latter drastically changes the electrical characteristics of the composition. Thus, the metal filler in the polymer matrix used for the known composition contributes to a rapid increase of the specific resistance of the composition in the electrolytic medium and stipulates its instability to anode dissolution. As a result, the insufficient vibration and frost resistance, as well as the low flexibility of the material based on the known composition make it practically inapplicable for the grounding electrode.
Known in the art is an electrolytic composition for coating extended conductors which comprises in weight per cent: electrically-conductive filler (calcined coke) 5-7%; polymer binder (ethyl lithacrylate and other acryl-latex polymers in emulsions) 5-50%; water-based solvent 5-50%; surface-active additive 0-5%; thickener 0.1-10%: alcohols C.sub.3 -C.sub.12 0.01-2.5%; a compound containing a bacterial anti-corrosion protective substance and fungicides 0.01-2.5% (U.S. Pat. No. 4,806,272).
The composition is used in the form of an electrically-conductive coating for cathodic protection against corrosion of steel structure of reinforced concrete members.
However, the known composition has inadequate electrical conductivity and low resistance to anode dissolving due to weak hydrolytic stability of carboxyl groups, their liability to moisture absorption and this increases the anode dissolution. Thus, the life of the coating based on the known comsition is low. In addition, the coating based on the known composition has insufficient elasticity due to inadequate elasticity of the acrylates and the absence of reaction of the coke with a polymer of the acrylate type.
The known composition can be used only in the form of an anode layer on a cathode polymerizable structure and cannot be made in the form of grounding electrodes of the pin or cable type using traditional process equipment, and this limits the field of applciation of the composition and makes it unsuitable for protection of elongated underground metal structures.
The closest in technical essence to the claimed composition is that for a long-line flexible electrode used in systems for anti-corrosion cathodic protection of metal objects, e.g. pipelines. The composition comprises the following components in wt. %: an electrically-conductive filler (gas soot or graphite) 23-55; a polymer binder (thermoplastic polymer, polyvinyl isenfluoride and acryl resin, chlorinated polyethylene) 65-44.8; additives (antioxidant, calcium carbonate) 0.1-5.0. The specific resistance of the composition is 0.6-29 ohm cm at 23.degree. C., its relative elongation is 10% (GB, A, 2100290).
From the point of view of possible application of the known composition in grounding electrodes for cathodic protection of underground structures, it has a number of drawbacks. In the first place, this composition has low resistance to anode dissolution due to the tendency of hydrolysis of the components such as chlorinated polyethylene, polyvinylidene fluoride used in its binding matrix, and, therefore, moisture saturation in the composition material under the effect of ground electrolytes. In the second place, the plastic materials which are the base of its polymer matrix are not material consuming, i.e. the filler content is limited. As an inevitable result, the filler is washed out and this drastically increases the specific resistance of the composition, i.e. the necessary electrical characteristics of the protection circuit will be lost. In addition, the field of application of the known composition is limited due to its frost resistance. The low frost resistance is due to the fact that in all embodiments of the composition its binding matrix includes a polymer component (thermoplastic polymer, chloride or fluoride) comprising polymer links which have an elevated crystallization temeprature. Thus, the strength and electrical characteristics of the composition drastically deteriorate at low temperatures.
A significant drawback is also low plasticity of the composition (relative elongation is equal to 10%) and, therefore, low flexibility and low fatigue strength of the composition material. Electrodes based on the known composition have low resistance to cyclic strains which always occur during transportation and assembly.